Lol. You're not seriously saying that incest depicted in a fictional work of media using cartoon characters is illegal, are you? I almost feel like you're trolling here. But anyway, even more so than incest, murder is very much frowned upon and illegal as a rule of thumb everywhere. And yet, we see it in movies, art, books, games, and so forth all the time. Why? Because those depictions -- murder or incest and other dark themes -- in those media aren't actually happening in the real world. Game of Thrones anyone that was popular for a time? Lots of things have them such as any number of police procedural shows and very often in much starker, darker content than most things found on F95 and in other adult-oriented games. Simply put, Steam doesn't disallow such content. It's fine on there.
On Patreon, their TOS have moved further away from that scale per their preferences for content providers, but it isn't outright prohibited. It's better if providers avoid them especially if they thought about making such things central to their projects, but Patreon's Trust and Safety team would review projects that have been reported to them and will make a determination to allow those projects to continue on their platform if those themes are heavily central to the project in question (e.g., Eva's previous game GGGB). (In our example, it passed, btw.)